Firefox is fantastic as a browser, but a terrible environment for creating
and debugging JavaScript. The usual workflow for testing a new snippet is:

Open your editor

Write some HTML.

Include some <script src="..."> tags.

Write some JavaScript

Switch to browser, reload. Nothing happens.

Force reload. Aah, there we go.

Notice erroneous behavior

Edit JS.

Reload... no, damnit, force reload.

Firebug takes that down to:

Type some JavaScript

I am tempted to wax poetical about this for longer, but I can explain it
better with a screenshot than words:

To be clear: expressions and statements are executed in the page's context,
with chrome's protection level. (That means you can do stuff like turn off
the menubar and fullscreen the window from this prompt, functions not
generally available to page scripts). You can directly inspect your existing
JavaScript objects without having to resort to proliferation of alert()
statements. As the ">>>" prompt may imply, it works a
heck of a lot like Python's interactive interpreter.

This extension really is an absolute must-have for any web developer. It has
been critical in improving and debugging Athena.